STRAIN ON THE PHYSICAL PROPERTIES OP MATTER. 829 
It would appear that permanent torsion has much more effect than permanent 
extension in increasing the internal friction. The torsional rigidity of the metal is 
also much more diminished by permanent torsion than by permanent extension, but 
the effect on the rigidity is nothing like so great as that on the internal friction, 
which last amounts to nearly 7 6 per cent. 
Having shown that when sensible permanent deformation is produced, either by 
traction or torsion, there is an increase of internal friction, I will now consider the 
effect of stress, which is not sufficient to produce sensible molar permanent deforma¬ 
tion, but which causes molecular deformation. 
Experiment XIX. 
An annealed iron wire 601 centims. in length and 0'960 centim. in diameter. 
Attached to the bar of the vibrator was a pan, which, together with the bar, weighed 
rather more than 2 kilos. The wire was, after adjustment, allowed to remain for five 
days, and was in the meantime frequently vibrated. After this period weights were put 
into the pan, and after each addition of load, followed by a rest of about ten minutes, 
the logarithmic decrement was determined; before each determination the wire was 
suffered to make about 100 vibrations. 
Load in til os. 
Logarithmic 
decrement due to 
internal friction. 
Remarks. 
2 
•003241 
4 
•002726 
6 
•002231 
8 
•001406 
2 
•001492 
4 
•001246 
6 
•001606 
8 
•001346 
10 
•002377 
12 
•004177 
14 
•005563 
14 
•002441 
After a rest of 30 minutes. 
14 
•002366 
Started again. 
14 
•005606 
After having been vibrated for about ten minutes 
through an angle of 60°.* 
14 
•001268 
After a rest of 20 minutes. 
An additional load of 1 kilo, was now put on and caused a slight permanent exten¬ 
sion, which, after a rest of two days, was ascertained to be 2*6 centims. The wire was 
then reduced to its original length, and, 14 kilos, being the load, was allowed to vibrate 
for ten minutes. After this the logarithmic decrement was again determined. 
* Notice the “ fatigue of elasticity” (see p. 832). 
